This invention pertains to the art of blood pumps and more particularly to rotary blood pumps having a purge-type seal. The invention is applicable to modulation of purge flow in rotary blood pumps and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously applied in other blood pumps beyond the embodiments specifically described below wherein it is desirable to control the flow of purge fluid.
Rotary blood pumps are well known in the art as exhibited by the following U.S. Pat. Nos.:
Reich, et al.--4,135,253 PA1 Wampler--4,625,712 PA1 Moise--4,704,212
Each of these has in common that a shaft seal is purged with a fluid that flows from a non-blood side to a blood side of the seal. This use of purge fluid reduces thrombus formation and clotting at the seal interface, carries away heat, and precludes blood from entering the mechanism behind the shaft seal.
A key technical problem in the design of such purged seals is to maintain an acceptable flow rate of purge fluid through the seal. If the flow rate is too low, a mixing phenomenon can result in blood flow across the seal to the non-blood side, even as purge fluid flows in the opposite direction toward the blood side of the seal. Additionally, under low flow rates, cooling and flushing of the seal interface is poor, which can result in deposition and problems associated therewith.
On the other hand, if the flow rate is too high, a patient may encounter fluid overload problems. This is still a potential problem even if the purged fluid is biocompatible. Yet another consideration with high purge flow rates is the maintenance of an adequate supply of purge fluid.
In the patents referenced above, there is no structural arrangement to adjust the pressure or flow of purge fluid in response to changing conditions. Unless external controls are included, the blood pump is configured with either a low limit or high limit under consideration. Blood pumps designed around a low limit provide an adequate minimum flow and contemplate that an acceptable high limit will not be exceeded. Conversely, blood pumps designed toward the high limit expect that purge flow will never fall below the low limit. Clearly neither arrangement is suitable.